1. Field of the Invention
The present invention relates to interference metrology of polarized light for testing of optics. In particular, the present invention relates to a phase-stepping point diffraction interferometer and method which effects phase-stepping by linear translation of a Wollaston prism in a lateral direction to the optical path, and which utilizes a spherical reference wave for producing high contrast interferograms.
2. Description of Related Art
It is desirable to improve on the accuracy of techniques for etching structures into SiO2 to a desired depth by monitoring the etch depth in real time. It has been observed that the SiO2 etch rate in Buffered HF vary from 14 to 17 nm per minute. For a 384 nm etch depth, this can become an 82 nm etch depth uncertainty. It is desirable to reduce this uncertainty to less than 10 nm.
Previously, the present inventors have etched to approximately 80% of the desired depth, removed the mask in the region of interest, measured the current etch depth and recalculated an expected end point at the current etch rate. The SiO2 etch was then completed and its depth was measured at a different inspection point, as was done for the 80% point, and then again to confirm the expected etch depth result. This proved to be a time-consuming and labor-intensive process requiring extensive handling of the part. It is desirable to eliminate these steps and etch to a consistent, repeatable endpoint by monitoring the etch depth in real time.
All currently available step height measuring instruments require dry first surface access. It is preferable that a real time monitor be provided that can view the surface being etched from the back side. No instrument maker has done this to date. Also, the reflectivities of the surfaces being etched are not large due to the effective index-matching of water and optical surfaces.
An interferometric instrument is desired that will reduce the work required to insure SiO2 etch depths are done accurately to within +/xe2x88x925 nm, while improving process yield and reducing fluid consumption and the concomitant hazardous waste disposal costs. Slow degradation of the etchant solution as it is consumed is not of consequence if the etch rate is monitored, and thus the etchant remains viable for many cycles and waste disposal costs are significantly lowered.
Furthermore, it is also desirable for such interferometers and interferometric instruments to produce high contrast interferograms which enable precision measurements. To this end it is desirable to utilize a xe2x80x9cperfectxe2x80x9d reference wave; that is it should be simple and well characterized, such as a spherical wave. Perfect reference waves are typically produced by passing light through a small aperture, such as a pinhole or slit. One example of a point diffraction interferometer using a perfect spherical reference wave is described in U.S. Pat. No. 5,835,217 to Medecki. Use of perfect reference waves can provide improved measurements in both etch monitoring applications described above, as well as testing of optics and optical systems. Optical systems in particular can be more confidently tested for aberrations when a perfect reference wave is used.
Additionally, it is also desirable to ensure precise control of the phase shifting process for even greater measurement precision and accuracy. Accurate phase shifting, however, is not easily accomplished. In particular, the common phase shifting method of translating mirrors in a direction of the optical path is difficult due to the exaggerated effects of even minute mirror motions. Various other methods and arrangements have been developed to facilitate and improve phase shifting. For example, in U.S. Pat. No. 4,906,852 to Nakata et al, a Wollaston prism and a quarter wave plate are used to shear light into two beams. A wedge glass is then used to vary the optical path difference (OPD) of one of the beams relative to the other to thereby impart a phase shift. Nakata is a representative example of many interferometer arrangements utilizing a Wollaston prism for the limited exclusive purpose of beam shearing. Specifically, it does not directly employ the Wollaston prism to impart a phase shift. Separating the beam shearing and phase shifting functions between a Wollaston prism and an independent phase-shifting module, however, can increase the cost and complexity of the interferometer without adding much value or significantly improving results.
The present invention provides a phase-stepping point diffraction interferometer utilizing a Wollaston prism to shear light into reference and signal beams, wherein the reference beam is transformed into a spherical reference wave to produce high contrast images for accurate fringe phase measurement.
Additionally, the present invention provides a phase-stepping point diffraction interferometer which translates a Wollaston prism in a lateral direction to an optical path in order to cause phase stepping.
The present invention also provides a Wollaston prism phase-stepping point diffraction interferometer wherein phase-stepping is mechanically advantaged due to increased stroke length of the laterally translated Wollaston prism.
One aspect of the present invention is a phase-stepping point diffraction interferometer utilizing a Wollaston prism to provide both image shearing and to effect phase stepping. The interferometer comprises a light source for directing light toward a test optic along an optical path, and a Wollaston prism for shearing the light into a signal beam and a reference beam. A diaphragm is provided with a pinhole through which the reference beam is directed for producing a spherical reference wave. The interferometer also comprises means for detecting an interference fringe pattern produced by recombining the signal beam and the spherical reference wave. The interferometer also includes means for translating the Wollaston prism in a lateral direction with respect to the optical path so as to cause phase shifting of the signal beam and the spherical reference wave. And the interferometer has means for measuring a phase value of the interference fringe pattern by phase shifting the signal beam and the spherical reference wave.
Another aspect of the present invention is for a method for interferometrically testing a test optic utilizing a phase-stepping point diffraction interferometer arrangement. The method directs light from a light source along an optical path toward the test optic. Additionally, the light is directed through a Wollaston prism to shear the light into a reference beam and a signal beam. The reference beam is directed through a pinhole of a diaphragm to produce a spherical reference wave. Next the signal beam and the spherical reference wave are recombined to produce an interference fringe pattern on a detector. Finally, a phase value of the interference fringe pattern is measured by translating the Wollaston prism in a lateral direction with respect to the optical path so as to phase-shift the signal beam and the spherical reference wave.